A major impurity of natural gas streams is toxic, flammable hydrogen sulfide (H2S), which must be removed to safely use natural gas. The removal of H2S, a process known as “gas sweetening”, employs physical or chemical sorbents. Gas sweetening with chemical sorbents is typically performed with aqueous solutions of alkanolamines such as monoethanol amine (MEA). However, gas streams purified in this manner must also be dehydrated to remove water so as to prevent formation of hydrates that trap methane. Presence of water can also cause corrosion, and “slug” formation in gas pipelines. Water also limits the effectiveness of the contaminant removal (or gas sweetening) process by increasing an already high thermal energy requirement for stripping H2S. Furthermore, high temperatures (80° C. to 105° C.) required to heat the aqueous alkanolamine solutions to drive the gas out of solution to regenerate (e.g., thermally swing) the solution require large amounts of energy, which can be responsible for thermally degrading the alkanolamine the aqueous alkanolamine solvent. And, while physical sorbents (e.g., SELEXOL® and RECTISOL®) can be used to sweeten gas without the typical problems associated with water, to work effectively, such sorbents: 1) often suffer from low capture capacities (<10 wt %), 2) typically absorb H2S under environmental (total) pressures greater than 100 psi, and 3) often perform poorly at gas pressures below about 20 bar. Thus, use of physical sorbents can involve a large capital investment and require large infrastructures to effect removal.
Hydrogen sulfide (H2S) is an acid gas that does not contain an electrophilic central atom and therefore its reactivity is different than that of other acid gases. As such, H2S is incompatible with traditional switchable ionic liquids that form alkylcarbonates and alkylsulfites because it doesn't react with alcohols and it reacts directly with strong bases such as amidine and guanidine to form thermally irreversible, solid hydrosulfide salts. Accordingly, new chemically-selective systems and processes are needed for stripping H2S that do not have the drawbacks of aqueous-based systems, or the high costs associated with conventional physical adsorbents and chemically-selective capture technologies.